1. Field of the Invention
The present invention relates to coating techniques and, more particularly, to a method of applying a coating by virtue of the energy of explosion for ejecting a powdered material towards a workpiece in recurrent cyclic operation.
This invention can be employed for coating surfaces of various materials with a view to protecting them against corrosion and mechanical effects. Metals, refractory compounds and materials based thereon, such as hard alloys etc., can be utilized as a coating material.
2. Description of the Prior Art
In general, methods of coating by virtue of explosive energy, particularly the energy of detonation waves, comprise preparing explosive mixture composed of a combustible gas, an oxidizing gas (as a rule acetylene and oxygen), and a gaseous suspension consisting of a powdered coating material and an inert carrier gas, supplying the explosive mixture and gaseous suspension to a combustion chamber which is a part of an elongated barrel having a closed end, and igniting the explosive mixture after said chamber is filled and the supply of said ingredients is cut off. The explosive energy is transmitted to the particles of powdered coating material which are suspended in the gaseous explosive mixture. As a result of the explosion, the heated and accelerated particles are blown out from the open end of the barrel at a great velocity towards the workpiece. Upon striking the workpiece surface, the particles of the coating material form a coated spot.
During the coating process, devices for accomplishing said methods (usually referred to as "detonation guns") are kept stationary, while the workpiece, placed at a certain distance from the open end of the detonation gun barrel, is moved perpendicularly relative to said barrel axis by means of any suitable device, for instance a manipulator. The speed and direction of the workpiece movement relative to the open end of the detonation gun are determined by the detonation rate of the gun and the required thickness of coating. Alternatively, the workpiece may remain immovable while the detonation gun is moved perpendicular to, and at a certain distance from, the workpiece surface.
One of the main disadvantages which makes impossible a wide application of coating methods using detonation waves is the generally accepted way of preparing the explosive mixture for each individual cycle. To carry out this operation, a complex gas distributing apparatus is usually employed for cyclically filling a mixing chamber and/or a combustion chamber with portions of the explosive mixture.
During the cyclical coating process all the operations proceed within several milliseconds, and even the slightest difference in the time of opening of the valves, through which combustible gas and oxidizing gas are introduced, adversely affects the quality of the explosive mixture, that is, the optimum quantitative ratio of the components of the mixture may not be maintained. In addition, the above time interval may prove to be insufficient for homogenizing an individually prepared portion of the explosion mixture.
These factors disturb the stability of the coating process and worsen the quality of coating. Thus, a variable quantitative ratio of the components of the explosive mixture causes temperature fluctuations and changes in the ejection velocity of the detonation products and powdered coating material. For this reason there is observed a marked scattering of values of technical coating indices in each particular spot. Said disadvantage is increased even more due to the fact that particles of the powdered coating material are heated unevenly in each cycle as a result of the explosion heterogeneity of the explosive mixture. A method of coating is described in U.S. Pat. No. 2,950,867 which employs the energy of explosion for ejecting a powdered coating material towards a workpiece in a recurrent cyclic operation. This method comprises preparing portions of an explosive mixture of acetylene and oxygen within a combustion chamber of the coating apparatus, introducing a powdered coating material into said chamber in the form of a gaseous suspension which employs an inert carrier gas, and igniting the explosive mixture immediately after a portion of the powdered coating material has been completely injected into the combustion chamber.
The apparatus for coating by detonation waves, employed in carrying out the aforedescribed method, includes a complicated gas distributing apparatus and a combustion chamber having poppet valves connected to a cam mechanism provided for opening and closing the valves in accordance with the predetermined rate of operation.
All the aforementioned disadvantages are inherent in the above-described method. Thus, for instance, fluctuation of concentration of the ingredients of the explosive mixture amounts to 46% in various zones of the combustion chamber. The heterogeneity and variation of the quantitative composition of the explosive mixture for above reasons, cause heterogeneity of the coating structure and variation in the strength of the coating layer and, in some places, in adhesion between the coating material and the workpiece material.